Dedicated Android fans will probably be a little disappointed that the latest update to HTC's One M8 flagship on Sprint doesn't include the bleeding edge 4.4.4. But it's pretty close, 4.4.3, and Sprint has thrown in a couple of notable expansions specifically for its network. In addition to the small back-end improvements in 4.4.3, the 2.16.651.4 release adds the Wi-Fi Calling feature that Sprint has been slowly rolling out for the last few months.
Users should also see an icon with the HD Voice badge, which will make Jack Bauer happy, at least. Sprint hasn't mentioned any other changes, though there may be some of the usual improvements and bug fixes. I also wouldn't be surprised to see another update in a month or so to fix any issues that might pop up here. Sprint is a fan of the staggered rollout, so if you don't see the update in your statusbar already, it may be a few days. There's no word on the size of the download, and as usual, those who've rooted or otherwise modified their phones may not be able to do a straight flash.
Source: Sprint support